633. The ‘Ultimate Issue’ Rule. Even though a witness qualifies as an expert in a particular field, the law imposes limits on the admissibility of the evidence the expert may give. For present purposes, three such limiting rules need to be mentioned: the ultimate issue rule, the basis rule, and the common knowledge rule. The first of these is the so-called ‘ultimate issue’ rule, so called because it relates to the ultimate issue or issues of fact to be decided in the case. In Samuels v Flavel Chief Justice Bray discussed ‘the propriety of a witness, particularly an expert witness, giving evidence in terms of the ultimate issue to be decided by the court’. In his view:
Such evidence is probably still strictly inadmissible … though it is, in fact, often admitted without objection and there may be cases when it must be admitted … because it is impossible for the opinion of the expert to be conveyed in any other form. But in my view it should be allowed only where absolutely necessary …
It seems clear that this represents the law in Australia, although the rule is quite often evaded, if not ignored, in practice. This makes it difficult to predict when it will be applied to exclude what might seem to be highly relevant evidence. For these and other reasons the rule has often been criticised. It would be artificial and unhelpful to prohibit a witness from expressing a considered opinion as to Aboriginal customary laws or traditions, even though the opinion related directly to an ‘ultimate issue’. In R v Isobel Phillips, for example, the admission of opinion evidence as to the operation and content of relevant customary laws so as to give rise to a defence of duress, was not objected to by counsel, and was accepted by the court. The Commission is not aware of any customary law cases where such a restriction has been imposed. Whether this is because a direct expression of opinion on such matters is, in Chief Justice Bray’s terms, ‘necessary’ or simply because the rule is not applied in practice, is unclear. Comments by Justice Blackburn in Milirrpum v Nabalco Pry Ltd support the view that the rules of evidence do not preclude expressions of expert opinion on Aboriginal customary laws even where these relate directly to an ultimate issue in the case. He said:
A further objection to the evidence of the expert witnesses was that they tended to apply unwarranted concepts of their own to the actual facts of Aboriginal behaviour and to talk in terms of such concepts, even to the extent of expressing themselves in terms which anticipated the findings of the Court on the issues before it. It was maintained, for example, that questions and answers expressing the idea of the ‘rights’ of clans of Aboriginals to particular land were objectionable … In my opinion it is … far preferable to allow the expert to answer questions in terms of ‘rights’, ‘claims’, etc, provided that the Court at all times remembers that there are two questions which are solely for it to decide. The first is … whether the conclusion of the expert, be it expressed in terms of ‘rights’, etc, or not, is one to which the Court should come. This is a question of fact. In deciding it, the Court must be alert to the danger of allowing its conclusions to be unjustifiably affected by the use of words which are only tentatively appropriate … The second is whether what is tentatively called the ‘right’ can be subsumed under some category which enables it to be recognized at common law, for example whether it can be properly characterized as a right of property. This is a matter of law.
The objection to admissibility, though not based on the ‘ultimate issue’ rule, was closely related to it, and was rejected in terms that clearly imply rejection of the ultimate issue rule itself in such circumstances. This result is certainly desirable, and if Justice Blackburn’s views represent the Australian law there appears to be no special need for reform in the present context. But the significance of Milirrpum’s case is reduced because it was a civil matter. The tendency has been for the ultimate issue rule to be relied on by the courts to exclude evidence that might assume undue weight in the mind of a jury. The absence of a jury in civil cases removes this concern. This Commission, in its Evidence Report, recommended abolition of the ‘ultimate is sue’ rule in federal courts. This recommendation would not apply to State or Northern Territory courts, and it is possible that the ultimate issue rule might be an obstacle to testimony about Aboriginal customary laws, at least in criminal cases.
634. Opinions Based in Part on Hearsay: The ‘Basis’ Rule. One important limitation on expert evidence relates to expert opinion evidence based upon hearsay (that is, upon statements of another person the truth of which is relied on, and which do not fall within one of the exceptions to the hearsay rule). Experts, like all other witnesses, are bound by the hearsay rule. Expert evidence which is merely the repetition of hearsay would be excluded by that rule. But much opinion evidence, especially in the social sciences, is based upon an accumulation of data including hearsay. To exclude opinion evidence based in part on hearsay would be to exclude expert evidence in many fields of human behaviour. This applies particularly to expert anthropological evidence of Aboriginal customary laws. In the last analysis all anthropological opinion of an unwritten culture such as traditional Aboriginal culture is based upon a combination of observing Aboriginal life and events and talking to Aborigines about them. Moreover observation without explanation is of limited value: in matters of culture, tradition and customary law it is the explanations and beliefs of the participants (their ‘internal attitude’) which give form and meaning to their actions. It appears that the existing Australian law does not exclude altogether expert evidence based in part on hearsay. The High Court summarised the position in these terms:
Statements made to an expert witness are admissible if they are the foundation, or part of the foundation, of the expert opinion to which he testifies but … if such statements, being hearsay, are not confirmed in evidence, the expert testimony based on them is of little or no value.
Whether the law excludes unsupported expert opinion evidence which is based entirely or substantially upon inadmissible evidence is unclear. In criminal cases there is a discretion to exclude unsupported opinions of this kind, if led by the prosecution, on the ground that they are of no probative value or that their probative value is outweighed by their likely prejudicial effect. In civil cases, however, the lack of admissible evidence to support opinion evidence based on hearsay will usually go to weight rather than admissibility.
635. Opinions of Aboriginal Customary Laws Based in Part on Hearsay. It seems then that Australian law draws a distinction between the mere repetition of hearsay (which is inadmissible) and the expression of an opinion by a qualified expert based in part on hearsay (which is not inadmissible). The same distinction has been drawn, in the context of proof of customary laws, in overseas jurisdictions such as India, and by the Aboriginal Land Commissioner. It was also drawn in the leading case, Milirrpum v Nabalco Pty Ltd. There, a major objection to the expert evidence was its hearsay basis. Like the other objections, it was rejected. Justice Blackburn said:
I do not think it is correct to apply the hearsay rule so as to exclude evidence from an anthropologist in the form of a proposition of anthropology — a conclusion which has significance in that field of discourse. It could not be contended — and was not — that the anthropologists could be allowed to give evidence in the form: ‘Munggurrawuy told me that this was Gumatj land.’ But in my opinion it is permissible for an anthropologist to give evidence in the form: ‘I have studied the social organization of these Aboriginals. This study includes observing their behaviour; talking to them; reading the published work of other experts; applying principles of analysis and verification which are accepted as valid in the general field of anthropology. I express the opinion as an expert that proposition X is true of their social organization.’ In my opinion such evidence is not rendered inadmissible by the fact that it is based partly on statements made to the expert by the Aboriginals.
This decision was based partly upon acceptance of the ordinary methods of anthropology as a ‘valid field of study and knowledge’, and partly upon the unacceptable distinction that would otherwise exist between the natural and social sciences:
The anthropologist should be able to give his opinion, based on his investigation by processes normal to his field of study, just as any other expert does. To rule out any conclusion based to any extent upon hearsay — the statements of other persons — would be to make a distinction, for the purposes of the law of evidence, between a field of knowledge not involving the behaviour of human beings (say chemistry) and a field of knowledge directly concerned with the behaviour of human beings, such as anthropology … I do not believe that the law of evidence requires me to put chemistry into one category and anthropology into another.
Justice Blackburn preferred to view the matter as ‘one of weight, rather than of admissibility of the evidence’. This view is cited with apparent approval in the leading Australian text on the law of evidence, and it is consistent with the views expressed by the High Court. However the matter cannot be regarded as settled. In England a more restrictive rule may exist, at least in criminal cases, and in the Warumungu Land Claim, Justice Maurice expressed at least tentative disagreement with Justice Blackburn’s reasoning. It may also be that the courts would take a more cautious position on opinion evidence based on hearsay when the evidence was before a jury. Milirrpum was a civil matter.
636. The Common Knowledge Rule. Expert testimony on matters of ‘common knowledge’ is generally inadmissible. The courts have been reluctant to permit experts to testify on matters which juries are capable of assessing for themselves. However they are likely to accept that juries are not acquainted with Aboriginal customary laws, traditions and ways of life generally. and in fact this has been the case: the rule has not been applied to exclude anthropological evidence of Aboriginal customs, practices and traditions.
637. Conclusion: Expert Evidence. To summarise, the common law rules with respect to the qualification of persons as expert witnesses about Aboriginal customary laws, traditions and culture appear to be satisfactory. A possible exception is the case of expertise acquired by experience without formal study or qualifications, although it appears that the Australian common law is moving towards allowing sufficient experience of this kind to qualify a person as an expert witness. However potential difficulties in the proof of Aboriginal customary laws arise from the ultimate issue and basis rules. There is considerable uncertainty about whether these rules may exclude opinion evidence about Aboriginal customary laws which relate to an ultimate issue in the case, or which is based upon statements not proved in evidence, especially in criminal cases. Evidence of Aboriginal customary laws should not be excluded by the ultimate issue rule. The basis rule should be treated, in criminal as well as civil cases, as a matter going to weight rather than admissibility. A rule which required anthropologists to prove the basis of any opinion would create practical difficulties in many cases. In addition opinions may be based on material that cannot or will not be formally admitted, eg because of questions of secrecy. Insistence on an exclusionary rule would deprive the court of much valuable evidence. However, before considering whether and what reform is called for, the rules relating to the admissibility of Aboriginal evidence about their customary laws need to be discussed.